The dietary methionine requirement of juvenile Arctic charr Salvelinus alpinus (L.) was assessed by feeding diets supplemented with graded levels of 12, 15, 18, 21, and 24 g kg A1 dietary protein) for 16 weeks at 12°C. All diets contained 400 g kg A1 protein, 170 g kg A1 lipid, 66 g kg A1 ash and an estimated 17.5 MJ digestible energy (DE) kg A1 . When live-weight gain was examined using quadratic regression, the estimate of methionine requirement for optimal growth was 17.6 g kg A1 of dietary protein (DP) or 7 g kg A1 of the diet. Requirements estimated on the basis of carcass protein and energy gains were 18.8 and 17.9 g kg A1 DP, respectively. Plasma methionine concentrations and ocular focal length variability measurements did not provide a sensitive measure of requirement, because each responded in a linear fashion to increasing dietary methionine levels. Based on the prevalence of cataracts, the methionine level required to prevent lens pathology (26.7 g kg A1 DP) appears to be higher than that required for maximum growth.KEY WORDS:

Rainbow trout, Oncorhynchus mykiss (Walbaum), with a mean initial weight of 196 g were fed the β-agonist ractopamine incorporated into a commercial rainbow trout diet at 0, 5, 10, 20 or 40 p.p.m. for 12 weeks. The experiment was conducted as a completely randomized design, with five treatments and four replicate tanks. At the start of the trial and every 4 weeks subsequently, fish were euthanized, and blood serum and liver samples were collected. Serum glucose concentrations were significantly higher in fish fed 40 p.p.m. ractopamine for 8 weeks, while all treated groups demonstrated significantly elevated glucose levels relative to controls at the 12 weeks sampling interval. Nonesterified fatty acid (NEFA) levels were significantly higher in fish fed 5 and 20 p.p.m. ractopamine for 4 weeks, but those fed 10 p.p.m. displayed depressed NEFA concentrations compared to controls. At 4 weeks, plasma growth hormone (GH) levels were significantly lower (P Ͻ 0.05) in the 5, 20 and 40 p.p.m. groups compared to controls. The results suggest that dietary administration of ractopamine to rainbow trout modestly alters the concentrations of circulating GH, glucose and NEFA.